This proposal is for the design and pre-prototype construction of an ultrasensitive ion source which can be easily interfaced to quadrupole mass spectrometers for real time trace constituent detection. The design will be based on the combination of preconcentration and recombination-limited low pressure Air Chemical Ionization (Air-CI) concepts. The gain in concentration of trace constituents generally depends upon the durations, pressures and gas flow rates of sampling and purge phases. Unlike a conventional preconcentrator which uses only time parameter to increase the concentration of constituents, the new design preconcentrator will utilize both pressure and time parameters. The concentrated trace constituents will be ionized by using an appropriate Air-CI source which operates at a pressure of about 10- 30 torr. This unique combination of a preconcentrator and matching Air-CI source will result in an ultrasensitive ion source for mass spectrometers. We envisage the sensitivity of the ultrasensitive preconcentrator/ion source would be superior to that of existing API sources, and would thus facilitate ultra-low-level diagnostic measurements, toxicity studies and other laboratory and industrial applications not now possible.